It's no surprise for scientists that the summit of Mauna Kea, a dormant volcano on Hawaii's Big Island, was the choice for the Thirty Meter Telescope (TMT), one of a handful of next-generation optical telescopes that aims to propel ground-based astronomy in the 21st century.

For professional astronomers, rarified air and dizzy spells are a small price to pay for Mauna Kea's front-row seat on the cosmos. And it is no mystery why it is home to one of the world's largest collections of observatories, with 13 operated by scientists from 11 countries. "The nighttime sky at Mauna Kea is simply spectacular," says Michael Bolte, director of University of California Observatories and a member of the board that chose this summit as the site for the TMT. "When you go outside at night and look up, it is like looking through a window straight into the heart of the universe."

On a trip to the observatory sites, visitors quickly leave the lush tropics and enter a stark environment that NASA's Mars rovers might recognize. The oxygen thins, the temperature drops to freezing, and the sky deepens to dark blue. At 4,205 meters above the Pacific Ocean, the summit of Mauna Kea commands a stunning view of the world below—and the universe above.

Geoff Marcy, an astronomer at the University of California, Berkeley, says the thin atmosphere takes its toll: Although the stars are spectacular, it's actually hard to focus on them at Mauna Kea's altitude. "After taking a short walk for a minute, nearly everyone feels dizzy," he says.

Many of the world's principal observatories on the planet are situated atop Mauna Kea—and above 40 percent of Earth's atmosphere—including the W. M. Keck's twin 10-meter telescopes, operated by the California Institute of Technology and the University of California, and Japan's 8.2-meter Subaru telescope.

Mauna Kea's telescopes have advanced studies of galactic evolution, the formation of stars and planets, and our own solar system. Last April, the Gemini Observatory there released a photograph of the oldest object ever seen—a gamma-ray burst measured at a redshift of 8.2. The light captured in its mirror had traveled for more than 13 billion years, nearly since the birth of the universe.

Several sites, including Cerro Armazones, Cerro Tolar and Cerro Tolonchar in Chile's Atacama Desert, and San Pedro Martir in Baja California, Mexico, had been considered for the TMT, a project of Caltech, the University of California, and the Association of Canadian Universities for Research in Astronomy. The scope, projected to cost about $754 million in 2006 dollars, is expected to be operational by 2018.

The remote Atacama is home to the European Southern Observatory. The Giant Magellan Telescope, with its seven 8.4-meter mirrors, will be built at Las Campanas Observatory in the Atacama and see first light by 2018.

Site evaluations for the TMT began in 2001 with a team of scientists trekking to some of the most remote places on Earth. The researchers installed weather stations and a suite of other instruments at each site to measure the transparency of the atmosphere, turbulence, dust and other factors. They also pored over satellite images.

By last year, two finalists had emerged—in opposite hemispheres: Mauna Kea and Cerro Armazones.

In the end, Mauna Kea, actually the summit of a dormant volcano that rises 9,750 meters off the ocean floor—the highest island mountain in the world—edged out Cerro Armazones for several reasons, astronomers with TMT said. Among them: Mauna Kea, at 4,205 meters above sea level, is higher than Cerro Armazones, which is situated at 3,064 meters. With less water vapor above the telescopes at Mauna Kea, the quality of observations at longer wavelengths, such as in the infrared, are better. The mean temperature at Mauna Kea is lower, which means less background thermal "noise" for observations at two-micron wavelengths and longer. The temperature atop Mauna Kea rarely varies far from 0 degree Celsius, from day to night and throughout the year. That makes it easier and less expensive to keep a telescope and its mirror at a uniform temperature because expansion and contraction of the optics is minimized.

There are few spots on Earth that rival places like Mauna Kea and the Atacama Desert for observational astronomy. Proximity to the ocean gives them a steady flow of air above, minimizing turbulence. Their height reduces water vapor, maximizing the amount of light that reaches the telescope's mirror. Both atmospheric turbulence and water vapor are enemies of every astronomer.

When bolstered by adaptive optics employed to correct atmospheric blurring, the Mauna Kea site, it was concluded, would offer slightly better observing quality than Cerro Armazones. (TMT's design will rely heavily on an adaptive optics system that employs a computer-controlled deformable mirror that cancels out most turbulence.)

The level of light pollution was evaluated at all of the candidate sites, but at Mauna Kea today, artificial lights that would otherwise corrupt the summit's pristine skies are not an issue. The lowest point in the sky the TMT will be able to point to is 65 degrees from zenith—above the reach of existing light pollution, says Matthias Schoeck, a TMT researcher who led site evaluations. Nevertheless, the potential for man-made lights to invade the sky above Mauna Kea concerns astronomers, Schoeck says.

"It is…important to prevent it from getting worse, not just for TMT but for other observatories, as well," he says.

Mauna Kea's more northern location did not give it an edge over Cerro Armazones. There are advantages to having the next generation of large ground-based telescopes situated in both the Northern and Southern hemispheres, in order to study different parts of the Milky Way, for example. But many of the observations to be made will be at cosmological distances, far beyond the confines of our home galaxy. For those studies it is less important where a telescope is situated, astronomers say.

Working at Mauna Kea can inspire astronomers to become poetic about the sky above the mountain. "Mauna Kea allows us to peer back in time when the galaxies and stars were first forming in the universe," Marcy says. "In effect, Mauna Kea offers humanity a glimpse of our roots, before the savanna, before the sun, and before any stars were shining in the cold, dark universe."

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